Hydrocracking of hydrocarbon feedstocks is often used to convert lower value hydrocarbon fractions into higher value products, such as conversion of vacuum gas oil (VGO) feedstocks to various fuels and lubricants. Typical hydrocracking reaction schemes can include an initial hydrotreatment step, a hydrocracking step, and a post-hydrocracking step, such as dewaxing or hydrofinishing. After these steps, the effluent can be fractionated to separate out a desired diesel fuel and/or lubricant oil base oil.
Most petroleum and hydrocarbon streams derived from crude contain naphthene and aromatic compounds. Naphthenes and aromatics are considered undesirable in some product streams as they may degrade product quality.
Distillate fuels typically contain paraffins, naphthenes, and aromatics. For fuel quality parameters such as cetane number, gravity and emissions, paraffins are the most desirable components, followed by naphthenes, followed by aromatics. The least desirable are multi-ring aromatic compounds. While various refinery processes produce distillate fuels, these processes are typically limited in their capability to produce high quality distillate fuel and/or high yields of distillate fuel. For example, conventional hydrogenation processes saturate aromatic rings to naphthenes, thereby increasing the cetane number and increasing the API gravity (lower density). The disadvantage of hydrogenation alone is that naphthenes have generally lower cetane values and are more dense than paraffins having substantially the same number of carbon atoms. The greater density of naphthenes results in reduced volume of the distillate fuel blend relative to a composition containing similar concentrations of paraffins instead of naphthenes. Similarly, multi-ring naphthenes are generally more dense and have lower cetane values than single-ring naphthenes having substantially the same number of carbon atoms. In addition, naphthenes can be converted to aromatics via oxidation reactions. Since combustion of naphthenes in fuels occurs under oxidizing conditions, there is the potential for naphthenes to revert to aromatics under combustion conditions, thus further reducing fuel quality.
For lubricant basestocks, multi-ring naphthenes can present drawbacks with regard to viscosity index (VI) and oxidative stability. Multi-ring naphthenes tend to have poor VI compared to paraffinic molecules. In addition, such naphthenes typically have poor oxidation stability which is undesirable in a high temperature environment due to thermal breakdown and sludge formation. Aromatics generally have poor VI and oxidation stability properties as well. Multi-ring aromatics have very poor VI properties and are usually hydrogenated for environmental and health reasons.
Conventional hydrocracking catalysts that open naphthenic rings rely on high acidity to catalyze this reaction. Because hydrocracking with a highly acidic catalyst breaks both carbon-carbon and carbon-hydrogen bonds, the use of such a catalyst cannot be selective in just opening rings of naphthenic species without further cracking desired paraffins.
Conventional ring opening catalysts typically comprise at least one noble metal hydrogenation component dispersed on an acidic refractory oxide support. Some conventional processes employ supported iridium-containing catalysts for opening naphthene ring compounds. However, these catalysts are sensitive to high sulfur content feeds (e.g., >10 ppm sulfur).
The self-supported mixed metal sulfide catalysts of the present disclosure generally exhibit sulfur and nitrogen tolerance as well as high selectivity for ring opening of naphthenic compounds.